Everything works fine.
But I want it to behave such that when it is in a small screen, and there is room only for one column, the one that shows is the second column (1 in the array).
I tried { responsivePriority: 1, targets: 1 } in columns[] and also in columnDefs[]
It works in both.
The problem is that the child-row expand button does not display anymore. If I get the responsivePriority parameter, the button is back.